The above-mentioned packaging machines are well known from the state of the art and are for example so-called form-fill-seal packaging machines which form a plastic film into a tray. This tray is filled with the packaging item and subsequently closed with a cover plastic film. Another packaging machine is a so-called traysealer in which preformed trays are filled with a packaging item and then sealed with a cover film. The sealing of the cover film to the tray is in both cases executed by exposing the cover film and/or the tray to heat and pressing the cover film and the tray together. This pressure is for example applied by pressing an upper tool which is located above the cover film and a lower tool which is located below the tray together. In the past, it has happened often, that the upper tool and/or the lower tool have been damaged during the sealing operation, so that at least one of the tools had to be replaced and the packaging machine was not in operation for an extended period of time.